take the bit between the teeth, to

To begin to do something with decisive, stubborn resolve; to act in an bullheaded and unruly manner. A reference to a horse that is no longer under the rider's control because the bit (part of the apparatus used to guide it) is out of place in its mouth. The CEO started ignoring the board's demands and took the bit between the teeth, running the company the way she saw fit. In spite of my counsel, my clients took the bit in their teeth and went public with their accusations.

To be stubbornly self-willed; to push aside restraints and go one’s own way. The analogy here, to a horse that catches the bit in its teeth so that the rider or driver has no control over it, dates from the sixteenth century. John Lyly used it in his Pappe with an Hatchet (ca. 1589): “But if like a resty iade [restive jade, or nag] thou wilt take the bit in thy mouth, and then run over hedge and ditch, thou shalt be broken as Prosper broke his horses.” The expression is used less often today and may be obsolescent.